The present invention relates in general to adjustable hydraulic motors, and in particular to an axial or radial piston type hydraulic motor having at least two cylinder blocks with pistons cooperating with a crankshaft or with a guiding reaction member, a fluid flow distributor, intake and return pressure conduits between the distributor and the cylinders, and at least one pressure-actuated control valve arranged in the pressure conduit.
Adjustable hydraulic motors of this kind have the advantage that, at the same volume of flow, different rotary speeds of the motor can be obtained. For this purpose, the stroke volume of the pistons changes during each rotation. If high efficiency even at low rotary speeds or higher rotary moments are required, the radial piston type adjustable hydraulic motors are preferably employed. If radial piston type hydraulic motors are used where the pistons are in contact with an inner rotor, then the individual valves are arranged usually in the fluid distributing conduits between the cylinder blocks and the pressure fluid distributor and are operated by a switching pressure. When the pistons or cylinders of the motor are disconnected from the pressure fluid source by means of these control valves, then the motor is capable of operating in two or more different operational conditions. The control valves are arranged between the pressure fluid distributor and the cylinder blocks in such a manner that in a switching position "low volume stroke", then all or a group of the cylinders are connected to each other. In this condition, an exchange of pressure fluid takes place between the cut off cylinders and produces oscillation of corresponding pistons. Moreover, in order to neutralize kinematic irregularities and leakage, a connection between the cut off cylinders with the intake or return conduits or with the leakage collecting space is established.
A disadvantage of these known hydraulic motors designed for disconnecting the pistons is the loss in efficiency resulting due to the oscillating pistons and due to the exchange of pressure fluid between the disconnected cylinders. These losses tend to increase when the switched off pistons are continuously acted upon by the intake pressure. It is true that in this case the pistons are effectively forced against a crankshaft of a radial piston engine or against a guiding reaction member of an axial piston engine, but due to high intake pressures a relatively strong heating of the motor parts takes place, which in turn may cause the occurrence of excessive frictional forces which may lead to the danger of seizure of the pistons. If the switched off pistons are acted upon by the return pressure, then particularly in the case of high rotary speeds the pistons are susceptible to disengagement from the crankshaft or from the guiding track because the aforementioned pressing force is absent. As a consequence, the achievable rotary speed is limited. In addition, another disadvantage of the prior-art solution is the fact that, due to the continuous exchange of pressure fluid between the switched off cylinders, no fresh pressure fluid is admitted. The pressure fluid flowing back and forth in the disconnected cylinders is subject to a relatively strong heating which causes an additional loss in efficiency. Depending on the design and the degree of overlap of the control valves, excessively high pressure peaks in the leakage pressure or in the cylinder space may be generated when the switchover is made while the motor is running. Since the control valves usually have a negative overlap, there is a danger that, at low rotary speeds of the motor, no switchover of the cylinders is produced when the switching pressure collapses due to the leakage in the region of the overlap.